Deformation Twinning of Tin Single Crystals under Impact Loading

Abstract

The impact test has been carried out on the mechanical twinning of 99.98% pure tin crystals, the time of stressing being 10 -3 sec. The twinning occurred with a sharp yield process, when the [001] axis was nearly perpendicular (±15°) to the specimen axis. The upper yield stress scattered from 0.7 to 2.5 kg/mm 2 , while the lower yield stress and also the stress for the further thickening of the generated twin lamella were from 0.1 to 0.2 kg/mm 2 . Twinning occurred where large local stress was expected. The critical shear stress law approximately holds for the upper yield stress after indentation, the stress being 0.8±0.1 kg/mm 2 . Two explanations for the upper yield stress for twinning appear possible. In the first place, it may be the stress required to initiate the twin with the aid of the local stresses, and in the second place, it may be the stress required for the growth of the nucleated twin lamella across the crystal. It is likely that the growth of the twin nuclei is controlled by the critical shear stress law, and that the growth is suppressed by the appearence of slip.